Recently, at least the following trends have emerged in field of cellular telephony.
First, mobile broadband traffic has been exploding in wireless networks such as WCDMA (wideband code division multiple access). The technical consequence is a corresponding steep increase of the interference in these networks, or equivalently, a steep increase of the load. This makes it important to exploit the load headroom that is left in the most efficient way.
Second, cellular networks are becoming more heterogeneous, with macro RBSs (radio base station) being supported by micro and pico RBSs at traffic hot spots. Furthermore, home base stations (e.g., femto RBSs) are emerging in many networks. This trend puts increasing demands on intercell interference management.
Third, the consequence of the above is a large increase of the number of network nodes in cellular networks, together with a reduced operator control. There is therefore a strong desire to introduce more self-organizing network (SON) functionality. Such functionality may support interference management by automatic interference threshold setting and adaptation, for a subset of the nodes of the cellular network.
As a result, there are problems that can hinder providing efficient service. In WCDMA for example, the UEs (user equipments) may or may not utilize the power granted by the EUL (enhanced uplink) scheduler. This leads to an inaccuracy of the load prediction step, where the scheduler bases its scheduling decision on a prediction of the resulting air interface load of the traffic it schedules. This is so since the 3GPP (Third Generation Partnership Project) standard has an inherent delay of about at least 5 TTIs (transmission time intervals) from the scheduling decision until the interference power appears over the air interface. Also the WCDMA load prediction does not account for all imperfections in the modelling of an UL (uplink) radio receiver. This can lead to additional inaccuracies in the load prediction and estimation steps.
The inventors are not aware of any practical other cell interference estimation algorithm available that can provide other cell interference estimates with an inaccuracy better than 10-20%, and does so with close to transmission time interval (TTI, typically 2 ms (milliseconds) or 10 ms) bandwidth (typically 250 or 50 Hz) over interesting power and load ranges. As a result, it is not possible to make optimal scheduling decisions since the exact origin of the interference power in the UL is unknown.
Load Estimation without Other Cell Interference Estimation
Following is a discussion on measurement and estimation techniques to measure instantaneous total load on the uplink air interface given in a cell of a WCDMA system. In general, a load at the antenna connector is given by noise rise, also referred to as rise over thermal, RoT(t), defined by:
                              RoT          ⁡                      (            t            )                          =                                            P              RTWP                        ⁡                          (              t              )                                                          P              N                        ⁡                          (              t              )                                                          (        1        )            where PN(t) is the thermal noise level as measured at the antenna connector. For the purposes of discussion, PRTWP(t) may be viewed as the received total wideband power (RTWP) defined by:
                                          P            RTWP                    ⁡                      (            t            )                          =                                            ∑                              i                =                1                            I                        ⁢                                                  ⁢                                          P                k                            ⁡                              (                t                )                                              +                                    P              neighbor                        ⁡                          (              t              )                                +                                    P              N                        ⁡                          (              t              )                                                          (        2        )            also measured at the antenna connector. The total wideband power PRTWP(t), is unaffected by any de-spreading applied. In (2), Pother(t) represents the power as received from one or more cells of the WCDMA system other than an own cell, e.g. neighbouring cells. The Pi(t) are the powers of the individual users, e.g. UEs, of the own cell. One major difficulty of any RoT estimation technique is in the inherent inability to separate the thermal noise PN(t) from the interference Pother(t) from other cells.
Another specific problem that needs to be addressed is that the signal reference points are, by definition, at the antenna connectors. The measurements are however obtained after the analogue signal conditioning chain, in the digital receiver. The analogue signal conditioning chain introduces a scale factor error of about 1 dB (1−sigma) that is difficult to compensate for. Fortunately, all powers of (2) are equally affected by the scale factor error so when (1) is calculated, the scale factor error is cancelled as follows:
                                          RoT                          Digital              ⁢                                                          ⁢              Receiver                                ⁡                      (            t            )                          =                                                            P                RTWP                                  Digital                  ⁢                                                                          ⁢                  Receiver                                            ⁡                              (                t                )                                                                    P                N                                  Digital                  ⁢                                                                          ⁢                  Receiver                                            ⁡                              (                t                )                                              =                                                                      γ                  ⁡                                      (                    t                    )                                                  ⁢                                                      P                    RTWP                    Antenna                                    ⁡                                      (                    t                    )                                                                                                γ                  ⁡                                      (                    t                    )                                                  ⁢                                                      P                    N                    Antenna                                    ⁡                                      (                    t                    )                                                                        =                                          RoT                Antenna                            ⁡                              (                t                )                                                                        (        3        )            
To understand the fundamental problem of interferences from other cells when performing load estimation, note that:Pneighbor(t)+PN(t)=E└Pneighbor(t)┘+E[PN(t)]+ΔPneighbor(t)+ΔPN(t)  (4)where E[ ] denotes a mathematical expectation and where Δ denotes a variation around the mean. The fundamental problem can now be clearly seen. Since there are no measurements available in the RBS that are related to the other cell interference, a linear filtering operation can at best estimate the sum E[Pother(t)]+E[PN(t)]. This estimate cannot be used to deduce the value of E[PN(t)]. The situation is the same as when the sum of two numbers is available. Then there is no way to figure out the individual values of E[Pother(t)] and E[PN(t)]. It has also been formally proved that the thermal noise power floor is not mathematically observable in case there is a non-zero mean other cell interference present in the uplink (UL).
FIG. 1 illustrates a conventional algorithm that estimates a noise floor. The illustrated algorithm is referred to as a sliding window algorithm, and estimates the RoT as given by equation (1). The main problem solved by this conventional estimation algorithm is that it can provide an accurate estimation of the thermal noise floor N(t). Since it is not possible to obtain exact estimates of this quantity due to the other cell interference, the estimator therefore applies an approximation, by consideration of a soft minimum as computed over a relative long window in time. It is important to understand that this estimation relies on the fact that the noise floor is constant over very long periods of time (disregarding the small temperature drift).
One significant disadvantage of the sliding window algorithm is that the algorithm requires a large amount of storage memory. This becomes particularly troublesome in case a large number of instances of the algorithm is needed, as may be the case when base stations serve many cells and when techniques like 4-way receiver diversity is introduced in the WCDMA uplink. A recursive algorithm has been introduced to reduce the memory consumption. Relative to the sliding window algorithm, the recursive algorithm can reduce the memory requirement by a factor of more than one hundred.
Load Prediction without Other Cell Interference Estimation
Following is a discussion on techniques to predict instantaneous load on the uplink air interface ahead in time. The scheduler uses this functionality. The scheduler tests different combinations of grants to determine the best combinations, e.g., maximizing the throughput. This scheduling decision will only affects the air interface load after a number of TTIs (each such TTI is a predetermined time duration such as 2 or 10 ms), due to grant transmission latency and UE latency before the new grant takes effect over the air interface.
In a conventional SIR (signal-to-interference ratio) based method, the prediction of uplink load, for a tentative scheduled set of UEs and grants, is based on the power relation defined by:
                                                        P              RTWP                        ⁡                          (              t              )                                -                                    P              N                        ⁡                          (              t              )                                      =                                            ∑                              i                =                1                            N                        ⁢                                                  ⁢                                                            L                  i                                ⁡                                  (                  t                  )                                            ⁢                                                P                  RTWP                                ⁡                                  (                  t                  )                                                              +                                    P              neighbor                        ⁡                          (              t              )                                                          (        5        )            where Li(t) is the load factor of the i-th UE of the own cell. As indicated, Pother(t) denotes the other cell interference. The load factors of the own cell are computed as follows. First, note that:
                                                                        (                                  C                  /                  I                                )                            i                        ⁢                          (              t              )                                =                                                                      P                  i                                ⁡                                  (                  t                  )                                                                                                  P                    RTWP                                    ⁡                                      (                    t                    )                                                  -                                                      (                                          1                      -                      α                                        )                                    ⁢                                      P                    i                                                                        =                                                                                                      L                      i                                        ⁡                                          (                      t                      )                                                        ⁢                                                            P                      RTWP                                        ⁡                                          (                      t                      )                                                                                                                                  P                      RTWP                                        ⁡                                          (                      t                      )                                                        -                                                            (                                              1                        -                        α                                            )                                        ⁢                                                                  L                        i                                            ⁡                                              (                        t                        )                                                              ⁢                                                                  P                        RTWP                                            ⁡                                              (                        t                        )                                                                                                        =                                                                                                                  L                        i                                            ⁡                                              (                        t                        )                                                                                    1                      -                                                                        (                                                      1                            -                            α                                                    )                                                ⁢                                                                              L                            i                                                    ⁡                                                      (                            t                            )                                                                                                                                ⁢                                                                          ⁢                                                                          ⇔                                                                          ⁢                                                                          ⁢                                                            L                      i                                        ⁡                                          (                      t                      )                                                                      =                                                                                                    (                                                  C                          /                          I                                                )                                            i                                        ⁢                                          (                      t                      )                                                                            1                    +                                                                  (                                                  1                          -                          α                                                )                                            ⁢                                                                        (                                                      C                            /                            I                                                    )                                                i                                            ⁢                                              (                        t                        )                                                                                                                                ,                                  ⁢                  i          =          1                ,        …        ⁢                                  ,        I                            (        6        )            where I is the number of UEs in the own cell and α is the self-interference factor. The carrier to interference values, (C/I)i(t), i=1, . . . , I, are then related to the SINR (measured on the DPCCH channel (Downlink Physical Control Channel)) as follows:
                                                                        (                                  C                  /                  I                                )                            i                        ⁢                          (              t              )                                =                                                                      SINR                  i                                ⁡                                  (                  t                  )                                                            W                i                                      ⁢                          RxLoss              G                        ×                          (                              1                +                                                                                                                                                                                        β                                                              DPDCH                                ,                                i                                                            2                                                        ⁡                                                          (                              t                              )                                                                                +                                                                                    β                                                              EDPCCH                                ,                                i                                                            2                                                        ⁢                                                          (                              t                              )                                                                                +                                                                                                                                                                                                                                                        n                                                                  codes                                  ,                                  i                                                                                            ⁡                                                              (                                t                                )                                                                                      ⁢                                                                                          β                                                                  EDPDCH                                  ,                                  i                                                                2                                                            ⁡                                                              (                                t                                )                                                                                                              +                                                                                    β                                                              HSDPCCH                                ,                                i                                                            2                                                        ⁡                                                          (                              t                              )                                                                                                                                                                                                      β                      DPCCH                      2                                        ⁡                                          (                      t                      )                                                                                  )                                      ,                                  ⁢                                  ⁢                  i          =          1                ,        …        ⁢                                  ,        I                            (        7        )            
In (7), Wi represents the spreading factor, RxLoss represents the missed receiver energy, G represents the diversity gain and the β:s represent the beta factors of the respective channels. Here, inactive channels are assumed to have zero data beta factors. The beta factors hence represent the data power offset of the specific transmission.
The UL load prediction then computes the uplink load of the own cell by a calculation of (6) and (7) for each UE of the own cell, followed by a summation:
                                                        L              own                        ⁡                          (              t              )                                =                                    ∑                              i                =                1                            I                        ⁢                                                  ⁢                                          L                i                            ⁡                              (                t                )                                                    ,                            (        8        )            which transforms (5) to:PRTWP(t)=Lown(t)PRTWP(t)+Pneighbor(t)+PN(t).  (9)
Dividing (9) by PN(t) shows that the RoT can be predicted k TTIs ahead as:
                              RoT          ⁡                      (                          t              +              kT                        )                          =                                                                              P                  neighbor                                ⁡                                  (                  t                  )                                            /                                                P                  N                                ⁡                                  (                  t                  )                                                                    1              -                                                L                  own                                ⁡                                  (                  t                  )                                                              +                      1                          1              -                                                L                  own                                ⁡                                  (                  t                  )                                                                                        (        10        )            
In the SIR based load factor calculation, the load factor Li(t) is defined by (6). However, in a power based load factor calculation, the load factor Li(t) can be defined by:
                                          L            i                    ⁡                      (            t            )                          =                                            P              i                        ⁡                          (              t              )                                                          P              RTWP                        ⁡                          (              t              )                                                          (        11        )            and equations (8)-(10) may be calculated based on the load factor Li(t) of (11) to predict the RoT k TTIs ahead. An advantage of the power based load factor calculation is that the parameter dependency is reduced. But on the downside, a measurement of the UE power is needed.
In heterogeneous networks (HetNets), different kinds of cells are mixed. A problem that arises in HetNets in that the cells are likely to have different radio properties in terms of (among others):
radio sensitivity;
frequency band;
coverage;
output power;
capacity; and
acceptable load level.
This can be an effect of the use of different RBS sizes (macro, micro, pico, femto), different revisions (different receiver technology, SW (software) quality), different vendors, the purpose of a specific deployment, and so on. An important factor in HetNets is that of the air interface load management, i.e., the issues associated with the scheduling of radio resources in different cells and the interaction between cells in terms of inter-cell interference.
These issues are exemplified with reference to FIG. 2 which illustrates a low power cell with limited coverage intended to serve a hotspot. To enable sufficient coverage of the hot spot, an interference suppressing receiver like the G-rake+ is used. One problem is now that the low power cell is located in the interior of and at the boundary of a specific macro cell. Also, surrounding macro cells interfere with the low power cell rendering a high level of other cell interference in the low power cell which, despite the advanced receiver, reduces the coverage to levels that do not allow coverage of the hot spot. As a result, UEs of the hot spot are connected to the surrounding macro cells, which can further increase the other cell interference experienced by the low power cell.